Test Methods

Armor Tower, Inc. engineers are highly experienced at
performing foundation investigations. Here's a summary of
the test methods used when Armor Tower conducts a foundation
investigation:

Sonic Echo / Impulse Response Test

The Sonic Echo/Impulse Response (SE/IR) test is typically
used on caisson type foundations for caisson length
determination. This method requires that some portion of the
top or upper sides of foundation be exposed and smooth so
that the receiver can be attached. This requirement is for
source and receiver attachments, which are used to generate
and measure the wave energy that travels down the foundation
and reflects back from the bottom depth of the foundation and
or any discontinuity along the length of the foundation.

Clear echoes from the bottom depth with no shallower
reflections would indicate the foundation length based on the
reflection time, and also indicate that the foundation is
sound with no defects and voids. The maximum testable length
for foundations is typically 30 times length of the
foundation diameter (i.e. a 2-foot diameter foundation is
testable out to about 60 feet of embedded length
maximum).

A wave velocity of 12,500 ft/sec, which is an average
velocity of waves in concrete, is used to determine the depth
of the caisson foundation. The SE/IR test method provides a
depth accuracy of about 5-10% and relies on reflections from
a change of impedance between two interfaces.

If there are significant impedance changes in a pier
foundation, they are manifested as shallower echoes in the
SE/IR results, otherwise the reflections that are present in
the records would be from the bottom of the pier (caisson).
Depending on the velocity contrast between the contact and
the bedrock and the penetrations into bedrock and
bedrock-like soils, the bottom echo reflections may be weak
in the SE/IR records.

Parallel Seismic Test

The Parallel Seismic (PS) method requires the use of a
cased borehole located within a few feet of the structure to
be tested. The ideal case is a borehole that is parallel to
the structure and located only 2-3 feet away from it.

The borehole is drilled to a depth of approximately 10-15
feet deeper than the expected depth of the structure. The
borehole must be cased, and the internal diameter of the
casing should be no smaller than 2.0 inches. The casing then
is grouted in accordance to the appropriate portions of ASTM
4428. The bottom of the casing is capped, and the casing is
filled with water. (Typically a 2 inch ID PVC pipe is used
for this purpose).

The testing is performed using 24-channel seismograph
(Geometrics Geode) coupled to a 24 component hydrophone
string. The seismograph is microprocessor-controlled and
records data digitally.

The PS test requires hitting the exposed portion of the
structure (it could be a helical pier or an embedded pole)
while monitoring response at many locations in a borehole.
The time of transit of a sound wave is the basic measurement
- and the path of the sound wave is from the top of the pile
cap, thorough the pile cap, along the pile, through the soil
between the pile and the hydrophone, to the hydrophone. The
source of the sound wave remains at the top of the pile cap,
and the hydrophone assembly is set at successively greater
depths. The measurement resolution for this method is
typically about one to two feet.

Foundation Estimates

When no construction information is available for review,
reasonable estimates of as-built conditions may be developed.
Soil boring and downhole metal detectors are used to locate
the size of the footing and to estimate for reinforcing
steel.

For more information on Foundation Investigation or to get
a quote, please contact
us.